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GIS approach to statistical modelling for mineral deposits in the Singhbhum copper belt, Bihar, India, using geological and geophysical parameters
Background Geological and Geophysical Information:
Before
proceeding to the analytical part of the work, it is necessary to give a
geological and geophysical background of the terrain under scrutiny.
Geologically, the area forms a part of the Proterozoic Singhbhum Mobile
Belt (C.2300-2400 Ma) bordering the northeastern part of the Archaean Singhbhum
- North Orissa Iron Ore craton. The cratonic block consists of
granite-greenstone assemblage comprising both intrusive and volcanic rocks of
different phases. The Lower Proterozoic Dhanjori basin comprising
volcano-sedimentary rocks occur between the granites in the south and Lower
Proterozoic metasediments and metabasics of Singhbhum Group in the north. The
Singhbhum Shear zone runs close to the interface of the Dhanjori Group and the
Singhbhum Group of rocks and is associated with a host of minerals. All the
group of rocks of this belt uniformly dip towards north at moderate angles.
Three phases of deformation have affected the rocks of the shear zone.
High-grade granulite facies has been attained in the rocks lying north of the
shear zone while the rocks south of it have yielded low-grade greenschist facies
metamorphism.
The area of study for the present statistical analysis is
kept restricted between Baharagora in the south east Tamadungri in the west
along the shear zone having an average width of 18km (Fig.2) in view of
consistence in the raw data availability and the occurrence of known mineral
deposits with mining activity.
Prospecting in different parts of the
Copper belt has revealed that copper sulphides occur in almost all types of
rocks in the shear zone. These can be broadly grouped under the following
categories: (1) Metasediments e.g. quartzite, mica schist etc., and their
derivatives; (2) Metabasic rocks; (3) Soda granite or other granitoid rocks and
(4) Meta-ultrabasic rocks and their derivatives.
The mode of occurrence
of the sulphides as noted by different workers are: Massive veins, braided
veins, stringers, composite veins, dissemination, discordant irregular bodies,
sheet like bodies and branching and interconnected en-echelon lenses and layers.
Various authors suggested different structural elements for control of
mineralisation. The shear zone/thrust zone is the structural control for
localization of ores (Dunn, 1937), a set of cross fold is the controlling
structural element (Narayanaswami, 1959). It was suggested, Sen Gapta
et.al.(1961); Sen Gupta, (1965,1972) that ultimate control for localisation of
sulphide minerals, in all scales, was by two planner structures described as
gently dipping 'slip plane' (first designated as 'S1' and later as 'S5') and
steeply dipping 'cleavage' or 'schistosity' (first designated as 'S' and later
as 'S2').
Information on surface indication, host rock, structural set
up, mode of occurrence, control of mineralization, mineral assemblage,
paragenesis, geochemistry, lithology etc. for certain locations were collected
from data compiled by Anon, GSI, ER (1991) on data sheets shown in Table-2.
These data sheets formed the basis for building up the computerized database.
The reserve of copper and their grade in different mines and prospects
as compiled from the reports submitted by Robertson Research, Australia to HCL
are given in Table-1. This information was utilized for calculating the total
metal accumulation in these locations.
Geophysical Studies:
Both airborne and ground geophysical surveys have
been carried out extensively in this belt. The geophysical data of the area have
been collated from the airborne geophysical survey report of Project 'Operation
Hard Rock', GSI, AMSE (1968), published gravity map of NGRI (1981), gravity -
magnetic studies of Pathak, et.al., GSI (1989-90), and report on 'Project -
Singhbhum', GSI, ER (1991).
The airborne geophysical survey was done
employing magnetic (TF), electromagnetic and radiometric (total count) methods.
Ground geophysical surveys include magnetic (VF), gravity, electromagnetic, IP,
SP and resistivity methods.
The airborne magnetic map reveals that a
chain of low amplitude high intensity magnetic closures follow the trend of the
copper mineralized zone. The area occupied by Singhbhum Granite is outlined by
3000 gamma contour. The mapped outcrop of soda granite/feldspathic schist is
conformable to the trend of aeromagnetic closures. The radioactive zones have
been found to be in excellent correlation with the known mineralized zones. Both
radiometric and magnetic response are continuous along the strike extension of
Turamdih, Dhadkidih and other prospects, which are related to uranium and copper
mineralisation.
The area has picked up the usual negative gravity
anomalies. Two prominent trends of Bouguer gravity contours are indicated (i)
Over Singhbhum Group of rocks and Dalma volcanics having E&W trend and (ii)
over Iron Ore Group of rocks and Dhanjori volcanics with N-S to NW-SE direction.
The Dhanjoris are indicated as Bouguer high. The mineralized zones from
Kanyaluka to Rakha Mines have yielded linear high Bouguer gradient. This linear
'high' zone abuts against a gravity low (-39 m. gal) at Jaduguda, which is a
uranium producing sector. From Jaduguda the high Bouguer gradient continues upto
Nandup at the western end of the study area. Almost all the Copper Mines and the
mineralized zone of this belt are located within this feature of high gradient.
The feature that this high gradient Bouguer gravity does not follow the shear
zone in the west beyond Rakha mines upto Jaduguda may be explained by the change
in mineralogical milieu from a predominantly sulphide minerals at Rakha to
predominantly radiometric minerals at Jaduguda.
The ground magnetic (VF)
data is compatible with the airborne magnetic (TF) pattern. The economic Cu
mineralized zone from Badia to Rakha is reflected as an isolated low amplitude
magnetic highs in an axis. This axis corroborates with high gravity gradient as
discussed above. Another similar magnetic axis south of the earlier one has been
located over Dhobani extending along NW upto Dhanjori Pahar through Tamajhori,
Kasaidih and SW of Patkita. Recent exploration by GSI has proved economic
deposits at Dhobani-Tamajhori sector as footwall lodes. From this area onward,
this magnetic axis gradually turns southwest and finally almost merged with the
N-S gravity feature over Singhbhum Group of rocks. Both gravity and magnetic
components also show an excellent correlation in the far southeastward extension
of this belt around Kesharpur (Chaudhuri, et.al., 1996).
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